A satellite communication system may include a central hub and a plurality of terminals. The plurality of terminals may receive data transmitted by the hub and the hub may receive data transmitted by the plurality of terminals.
Traditionally, all terminals were stationary, i.e., remained at fixed locations at all times, including while receiving data (e.g., from a hub) and/or transmitting data (e.g., to a hub). In some cases, some terminals may have been transportable. A transportable terminal may include an antenna, which may be deployed and aligned with a satellite (either automatically or manually), but only when the transportable terminal is stationary. Therefore a transportable terminal may be operative only when stationary and thus it is in fact very similar to any ordinary stationary terminal having a similar size antenna.
More recently, satellite communication systems may also support mobile terminals. A mobile terminal may include a tracking antenna, which may maintain alignment with a satellite while the remote terminal is in motion. Thus, a mobile terminal may receive data (e.g., from a hub) and/or transmit data (e.g., to a hub) while on-the-move as well as when stationary.
The mobile environment introduces rapidly changing link conditions due to obstacles in the line-of-sight between the mobile terminal and the satellite (mainly applicable to land-operated-terminals, less to airborne or maritime terminals). Both received and transmitted signals may occasionally fade, sometimes considerably, and even become completely blocked, for example due to buildings, bridges or tunnels temporarily blocking the line-of-site between the terminal's antenna and the satellite.
There are many practical considerations affecting the sizes and shapes of tracking antennas for on-the-move satellite communication. These considerations may include, for example, the space available on a vehicle's roof, the weight of the tracking antenna, the profile of the antenna and the air flow disturbances it may cause around a moving vehicle (especially in case of an aircraft), as well as other considerations. All these considerations may cause certain tracking antennas for satellite communication on-the-move to be quite small (for example equivalent to dish antennas of 20 to 60 centimeters in diameter), and sometimes also to have a low profile (e.g., in order to minimize air resistance while on-the-move).
As an antenna becomes smaller, it has lower gain and a wider transmission/reception beam. Both of these phenomena may be undesired, as they make it more difficult to maintain communication with the terminal (e.g., due to the lower gain) and to avoid interferences to/from adjacent satellites (e.g., due to the wider beam). Since the antenna gain is in direct proportion to the antenna size and in reverse proportion to the wavelength of the transmitted and/or received signal (i.e., Gα(D/λ)2, where G stands for gain, D is the diameter of the antenna dish reflector and λ is the wavelength), one method for mitigating at least some of the effects of these phenomena is to use a higher frequency band, for example to use the Ka-band (˜30 GHz uplink, ˜20 GHz downlink) instead of the Ku-band (˜14 GHz uplink, ˜11 GHz downlink). However, the higher the frequency the greater are the effects of weather (e.g., clouds, water vapors, rain, snow, hale, etc.) on the path losses, hence it is harder to maintain the satellite link due to deeper fades.
Given all the above, in order to communicate using small antennas, e.g., for supporting on-the-move satellite communication, both a hub and a mobile terminal may need to transmit stronger signals (i.e., at higher power level) to overcome the deficiencies of the smaller antenna and of the higher frequency band, as previously described. However, this might not be possible, for example, due to regulations that may limit the spectral density (i.e., the amount of power per given bandwidth) of transmitted signals (e.g., in order to prevent interferences to adjacent satellites).
Spectrum spreading techniques (e.g., Direct Sequence, Repetitions, etc.) may be used to comply with transmission intensity regulations. These techniques may spread the total power of a signal over more bandwidth, thus reducing the spectral density of the signal below the limit allowed by the applicable regulations. However, as the spread signal requires more bandwidth, the total efficiency of a system using spreading may declines, as less user bits may be transmitted over a given bandwidth.
In a satellite communication system comprised of at least a heterogeneous population of terminals, some of the terminals may have relatively larger antennas (e.g., due to being stationary or due to being mobile but installed on ships or trucks where sufficient room exists and aerodynamic requirements are less acute), while other terminals may have very small antennas (e.g., due to being installed on aircrafts or on other vehicles that may have strict special and aerodynamic requirements). Given the differences in antenna sizes, terminals having larger antennas may use more efficient carriers (i.e., carriers that transmit and/or receive more bits per given bandwidth) while terminals having small antennas may be compelled to use more robust carriers and/or spreading techniques.